You can install adjusters for the front torsion bar anchor points (or just cut & turn them ~¼")...the springs are already slightly stiffer than those in a Beetle, but could be replaced with Sway-a-Way's parts (kinda spendy). Raising the rear on a swingaxle is going to induce positive camber, which can adversely affect cornering and make it easier to roll, so I'd advise bring conservative there, but the bars are splined and can be reindexed. It's labor intensive, but there's no cost involved...most folks replace the rubber bushings while it's apart, though.
Does the German military vehicle use 14" rims? 15" with some taller tires would help both ground clearance and highway cruising speed.

There is a limit to the droop and compression angles before they can do damage to the transaxle itself. Also "tuck under" can make your very nice expensive Thing do nasty, dangerous, self destructive things.

The Thing has a better designed ball-joint (BJ) design that sits about 3" higher and the BJs are pressed in both facing down so that the lower BJs are harder to pull out than with a bug. The rear shock mount on the Thing (not real sure about the swing arm though) I think is installed higher than a bug which allows it more clearance in the rear.

VW did good so you can go out and enjoy it off road just keeping in mind that there are limits to what you can do. There are a lot of people with "toys" that have Ball-joint beams and 2-joint (swing) rear axles that are having a great time off-road so don't feel bad... besides, you have a "Thing" which a lot of people would just like to have.